Moles & Formulas

Moles

By Lucila Giammatteo

Units of Elementary Entities

Relative Formula Mass

The mass of a compound relative to 1/12 the mass of an atom of carbon-12. Does not have any units.

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Use the periodic table  to determine the relative atomic mass or relative formula mass for the following elements and compounds.

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Determine Mr for each compound in the chart

Mr or Molar Mass?

Mr is adimensional, no units
Molar mass is defined as the mass in grams of 1 mole of a substance.

Law of Definite Proportions

For any given compound, the ratio of constituent elements is fixed. Also known as law of definite composition or Proust's law.

Percentage Composition by Mass

The relative amount for each element in a compound.

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Determine the percentage composition by mass of glucose, C₆H₁₂O₆.

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Determining chemical formulas from %

The percentage composition by mass of sodium chloride is 39.3% sodium (Na) and 60.7% chlorine (Cl). Determine the empirical formula of sodium chloride.

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A compound was collected and analysed to contain 25.94% nitrogen and the rest is  oxygen. Determine the empirical formula.

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The percentage composition by mass of aluminium chloride is 20.3% aluminium (Al) and 79.7% chlorine (Cl). Determine the empirical formula of aluminium chloride. The relative formula mass of the compound, M_r, is 267. Use this value to calculate the molecular formula of the compound.